[merge] Detect conflicting files underneath ignored folders
Fix the TreeWalk setup. Don't use the NotIgnoredFilter; it prunes
ignored directories top-down without checking whether there are
tracked resources underneath. Instead just set the DirCacheIterator
on the FileTreeIterator; it then does the right thing out of the
box: it skips ignored directories unless there are tracked files
underneath.
Bug: 483596
Change-Id: Ie062384655dfc4cedf40c32be95eeb133b6de954
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>